The effect of solution parameters on the thiosulfate leaching of Zarshouran refractory gold ore

• The presence of orpiment greatly affects the optimum conditions for gold recovery in thiosulfate leaching.
• Gold extraction is very low in the absence of EDTA.
• High copper concentrations, as well as ammonia, increase the oxidation of orpiment
• The released arsenic competes with the cupric ion for EDTA and decreases gold extraction.

In recent years, leaching of refractory gold ores has received much attention due to the depletion of oxide ores. Since the gold is usually associated with or encapsulated in sulfide minerals, such as pyrite, arsenopyrite and chalcopyrite, research in this field has focused mainly on these phases. However, the main sulfide mineral in Zarshouran gold mine (Iran) is orpiment, which has received very little attention. In this study, the effect of different additives on the thiosulfate leaching of gold from Zarshouran refractory gold mine (high-grade orpiment) has been investigated, by determining the thiosulfate and arsenic concentration in solution using UV–visible spectrophotometer and the copper and gold ions using atomic absorption.The results indicated that gold recovery in the presence of EDTA is higher than that achieved in the presence of ammonia, which is attributed to the stronger stabilization of Cu (II) and, consequently, the decrease in thiosulfate consumption. Increasing the copper concentration (greater than 0.05 mol/L) decreases the gold recovery, probably due to the increase in the solution potential, which causes higher thiosulfate consumption and, consequently, greater instability of the gold complexes. Although increasing the pH (mainly) and potential enhancement of the orpiment oxidation liberating the gold from the mineral, this also promotes copper and thiosulfate oxidations, which increase copper sulfide and copper oxide precipitation. Consequently, these actions decrease the gold dissolution (to less than 30%) by passivation of the gold surface. The maximum recovery achieved was 71%, with pH = 11, thiosulfate (0.5 mol/L), copper (II) (0.05 mol/L), EDTA (0.05 mol/L), after 24 h with no oxygen injection.